all: add internal/itoa package

This replaces five implementations scattered across low level packages.
(And I plan to use it in a sixth soon.)
Three of the five were byte-for-byte identical.

Change-Id: I3bbbeeac63723a487986c912b604e10ad1e042f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/301549


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>